Circuit boards are widely used in the electrical industry for radio, television, computers, appliances, industrial and electronic equipment. Printed circuit boards have been traditionally manufactured from a copper clad epoxy-glass laminate. When starting with this material, the shape of the printed circuit board must first be routed out and the holes for mounting the components (e.g., transistors, resistors, integrated circuits, etc.) individually drilled. The board is then masked with photoresist, the circuitry imaged, and the copper etched away from areas where it is not wanted.
Another procedure for manufacturing printed circuit boards involves injection molding the circuit board substrate with the holes in place. The molded substrate is then put through several adhesion promotion steps and plated with electroless copper according to standard technology, to produce the printed circuit board. In this injection molding procedure, the substrate material is limited to thermoplastic resins which resist blistering or distortion under the conditions needed for soldering. In wave soldering, currently the dominant method, one side of the plastic board briefly contacts an agitated pool of molten solder, and heating of the board is localized. In the emerging surface mount technologies, including vapor phase soldering (VPS), on the other hand, the entire plastic board is exposed to temperatures of at least 215.degree. C. (419.degree. F.). Obviously, savings result with these injection molded circuit board substrates due to the elimination of considerable mechanical processing such as routing and drilling.
The critical parameters of a printed circuit board, from a soldering standpoint, are its glass transition temperature (Tg), environmental stress crack resistance and thermal expansion coefficient. The higher a substrate's glass transition temperature (Tg) and environmental stress crack resistance to solder fluxes, the less likely it will blister or delaminate during soldering.
Other parameters of a printed circuit board are its plateability and resistance to water absorption. Acceptable plateability requires good adhesion of electrolessly plated copper to the circuit board substrate. Acceptable resistance to water absorption requires little or no water assimilation into the circuit board substrate so as to provide good electrical properties.
Poly(aryl ether)s containing the following repeat units: ##STR1## are commercially available thermoplastic polymers which have a wide variety of end-use applications. These thermoplastic polymers are described in, for example, U. S. Pat. Nos. 4,175,175 (formula 1) and 4,008,203 (formula 2). Such end-use applications include the use of these thermoplastic polymers for injection molding into circuit board substrates. However, a circuit board substrate molded from the poly(aryl ether) of formula (1) has a glass transition temperature (Tg) which is generally not as high as desired for soldering temperatures such as those encountered in wave soldering. Circuit boards molded from the poly(aryl ether) of formula (2) have an acceptable glass transition temperature (Tg) but do not have, in some instances, acceptable plateability and resistance to water absorption.
U.S. Pat. No. 4,520,067 describes a blend composition useful for making circuit board substrates and electrical connectors containing from 40 to 90 weight percent of a poly(ether sulfone), i.e., such as that of formula (2) above, and from 10 to 60 weight percent of a polysulfone, i.e., such as that of formula (1) above. A circuit board substrate molded from the blend composition is stated to have improved plateability.
U.S. patent application Ser. No. 448,376, filed Dec. 9, 1982, describes a blend composition useful for making circuit board substrates and electrical connectors, containing a poly(aryl ether), i.e., such as that of formulas (1) or (2) above, a poly(etherimide), fiber and a filler. A circuit board substrate molded from the blended composition is stated to have improved plateability.
U.S. patent application Ser. No. 011,898, filed Feb. 6, 1987, commonly assigned, describes select poly(aryl ether sulfone) polymers useful for molding into a circuit board substrate. Incorporation of specified amounts of hydroquinone in the synthesis of a polymer such as that of formula (2) above, produces a poly(aryl ether sulfone) which is suitable for being molded into circuit board substrates. When metal is electroplated onto such circuit board substrates, it is stated that there is a high degree of adhesion of the metal to the circuit board substrate.
U.S. Pat. No. 4,550,140 describes circuit board substrates prepared from poly(aryl ethers) which contain repeating units derived from bis(3,5-dimethyl-4-hydroxyphenyl)sulfone. These substrates are claimed to exhibit adequate glass transition temperatures (Tg), acceptable plateability and acceptable resistance to water absorption.